Fix and reword some exception messages.

lib/Predis/Connection/MasterSlaveReplication.php

@@ -44,9 +44,7 @@ class MasterSlaveReplication implements ReplicationConnectionInterface
     protected function check()
     {
         if (!isset($this->master) || !$this->slaves) {
-            throw new RuntimeException(
-                'Replication needs a master and at least one slave.'
-            );
+            throw new RuntimeException('Replication needs one master and at least one slave.');
         }
     }
 
@@ -148,7 +146,7 @@ class MasterSlaveReplication implements ReplicationConnectionInterface
             $connection = $this->getConnectionById($connection);
         }
         if ($connection !== $this->master && !in_array($connection, $this->slaves, true)) {
-            throw new InvalidArgumentException('The specified connection is not valid.');
+            throw new InvalidArgumentException('Invalid connection or connection not found.');
         }
 
         $this->current = $connection;